This invention relates to a method of manufacturing integral heated melt conveying nozzle probes for injection molding each of which have a removable thermocouple that extends down into highly conductive material adjacent the forward end to continuously measure the temperature in that area during use.
It is well known to provide injection molding systems with elongated probes, each of which projects into a melt passage along which the melt flows around the probe on the way to the gate. An example of this is shown in the applicant's U.S. Pat. No. 4,376,244 which issued Mar. 8, 1983. It is also well known to provide injection molding systems with elongated hollow nozzles or sprue bushings, each of which has a central passage through which the melt flows on the way to the gate. Examples of this are shown in the applicant's U.S. Pat. Nos. 4,238,671 which issued Dec. 9, 1980; 4,355,460 which issued Oct. 26, 1982; and 4,403,405 which issued Sept. 13, 1983. Both of these elongated probes and nozzles have internal electric heaters and in the present application the generic term nozzle probe is used to refer to them both.
As is also well known, close temperature control of the melt, particularly in the gate area, has become increasingly critical as more difficult engineering materials are being molded in more difficult applications. Cast and economical considerations have made systems having a large number of cavities which run more reliably desirable. Therefore, in order to improve the control of temperature in the gate area, it is advantageous to provide nozzle probes with the facility of improved temperature measurement at the forward end.
In view of the fact that thermocouples do burn out or malfunction relatively frequently, it is desirable that nozzle probes have a thermocouple receiving well or sleeve into which and from which the thermocouple is insertable and removable. This also avoids limiting the method of manufacture of the nozzle probe to steps which will not damage the thermocouple, which would otherwise be the case.
U.S. Pat. No. 4,120,186 to Crandell which issued Oct. 17, 1978 discloses a nozzle with a swaged construction having a thermocouple well which extends to the forward end and from which the thermocouple can be removed. However, the structure disclosed in each of the embodiments in Crandell shows the thermocouple or thermocouple well extending adjacent a compressible material. This has the disadvantage that it does not provide a measurement of the temperature at the forward end of the nozzle which is sufficiently accurate and reliable for materials which are difficult to mold because temperature requirements are critical.
Of course, the normal method of providing a well for a removable thermocouple is to drill it in the desired location. An example of this is shown in U.S. Pat. No. 4,424,622 to Krause which issued Jan. 10, 1984 and describes a method of drilling a thermocouple well through a plug into an injection molding manifold. While this method works very satisfactory for that application, it cannot be used for the present situation where close tolerances for the location of the well are essential. The problem is that the bore of the thermocouple receiving well must be very small in diameter because of the limited amount of space beside the heater member and the small size of the thermocouple itself. As a result, gun drills which are currently available are not capable of reliably drilling such small diameter wells or bores to the length required without the danger of wondering off line and thereby ruining the unit. Until the present invention, this difficulty has prevented the manufacture of nozzle probes with removable thermocouples extending into highly conductive material adjacent the forward end of a nozzle probe. As shown in FIG. 8 of the applicant's above mentioned U.S. Pat. No. 4,355,460, previously the thermocouple had to be cast in with the result that the whole unit has to be scrapped when the thermocouple fails.